B1791

Code

Other B — Body

Autolamp Sensor Input Circuit Open

Brand: Other

Type: B — Body

Views: UK: 20 EN: 30 RU: 21

AI status

Completed

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Page language: EN

Causes

Open or broken wiring between ambient light sensor and BCM
Disconnected or corroded connector at sensor or module
Failed ambient light (autolamp) sensor
Blown fuse or loss of power feeding sensor/module (if applicable)
Poor or missing ground at sensor circuit
Aftermarket work or previous repairs that damaged harness

Symptoms

Automatic headlamps not working (won't turn on or won't turn off automatically)
Dashboard message or lamp indicating autolamp fault
Headlamps remain on or off regardless of ambient light
Dimming/brightness control anomalies tied to ambient light input
Possible intermittent function of AUTO mode

What to check

Read and record freeze frame data and confirm B1791 is current or history
Visually inspect the ambient light sensor and surrounding area for damage or water intrusion
Inspect harness and connector for corrosion, bent pins, or disconnection
Check related fuses and power/ground circuits serving the sensor and BCM
Backprobe sensor connector to check for presence of expected signals and continuity to BCM
Wiggle harness while monitoring for change in signal or code status

Signal parameters

Typical ambient light sensor output: variable voltage signal in the 0–5 V range (vehicle-specific); lower voltage = dark, higher = bright
Open-circuit result: no voltage / infinite resistance on signal pin
Short to battery: near battery voltage on signal pin (~12 V) — not expected
Short to ground: ~0 V on signal pin — may be interpreted as continuous darkness
Resistance/continuity between sensor signal and BCM pin should be low (continuity); infinite if open

Diagnostic algorithm

Use a scan tool to confirm code B1791, note freeze frame and clear the code. Recreate conditions to see if code returns.
Perform a visual inspection of the sensor, mounting area, and wiring for damage, corrosion, or moisture.
Locate the ambient light sensor connector. With ignition ON, backprobe the connector and verify reference power (if present), ground, and signal voltage. Compare to expected ranges from service data (typically 0–5 V variable).
If signal is absent (open), check continuity between the sensor signal pin and the BCM connector pin. Repair any open circuit found (repair splice, replace damaged section).
Verify sensor power and ground circuits: check for blown fuses, presence of fused power, and good ground. Repair as needed.
If wiring and supplies are good but signal is invalid or intermittent, disconnect sensor and measure its output directly or substitute a known-good sensor if available.
After repairing or replacing wiring/sensor, clear codes, operate vehicle in AUTO conditions to confirm proper autolamp function and that the code does not return.
If wiring and sensor test good but code persists, consider BCM/lighting module diagnostics and consult manufacturer service data for module-level tests.

Likely causes

Damaged or disconnected sensor connector (water/corrosion)
Broken or chafed wiring causing an open circuit
Failed ambient light sensor element
Loose or corroded ground or power at sensor

Fault status

⚠️ Status

Autolamp sensor input circuit open. The ambient light sensor signal to the lighting control/BCM is not present or is open; automatic headlamp operation may be disabled or incorrect.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours